1) Field of the Invention
The invention relates to methods and apparatuses for the realization of frequency band-stop, band-pass and low pass filters as integrated circuit elements on a silicon chip for use in a quadrature modulator which can generate complex modulated radio signals.
2) Discussion of Related Art
The prior art for the construction of frequency selective filters includes: (1) passive inductor-capacitor filters; (2) passive resistor-capacitor filters; (3) active RC filters; (4) distributed RC filters; (5) gyrator-capacitor filters; (6) transmission line or waveguide filters; (7) switched capacitor filters; and (8) digital filters, each of which is discussed below.
The construction of inductor-capacitor filters on a silicon chip is constrained by the very small inductance values that can be achieved with spiral metallization patterns within a frequency range above about 2 GHz.
Passive resistor-capacitor filters can only synthesize a limited subset of the possible frequency responses that might be required, and at low frequencies are limited by the available RC product that can be made while also being limited at high frequencies by stray (parasitic) capacitance and resistance.
Active RC filters can provide useful performance up to a few megahertz, but are limited by the performance and bandwidth of amplifiers as well as the parasitic effects mentioned above. Unfortunately, the amplifiers consume power and limit the dynamic range.
Distributed RC filters are, on the other hand, inherently based on the parasitic capacitance and resistance parameters, such as described in "Tidskontinureliga Lagpass Filteri CMOS", by Katarina Hansson and Mats Torkelsson, LUTEDX/(TETE-7029)/pp. 1-26 (1987).
Gyrator-capacitor filters use an active impedance inverting circuit to make a capacitor function as an inductor, so that LC equivalent filters may be built. These circuits are useable for bandpass filters up to a few megahertz. The Gyrator-Capacitor filter can be classed as a form of active RC filter.
Transmission line or waveguide filters require elements that are typically a quarter wavelength long so their construction on a chip is limited to the micro-wavelengths above 2 GHz.
Switched capacitor filters operate according to a number of different principles, but all require transistor switches to operate at a very much higher frequency than the operating frequency range of the filter. This restricts their use to a few hundred kilohertz. Moreover, the dynamic range of switched-capacitor filters is limited by their high noise levels.
Digital filters are very flexible in the frequency response repertoire they can realize, and have the advantage of no tolerances. On the other hand, the signal to be filtered must first exist in digital form and the required analog-to-digital convertors restrict both the dynamic range and speed. Digital logic power consumption is also a factor which restricts such filters to the 300 kHz region or below in practical applications.
The frequency range upon which the present invention focusses is the 0.3 MHz to 300 MHz region. This is above the range of most of the techniques mentioned while being below the range for transmission line solutions. Hitherto there has been no practical silicon-integrable solution for these three decades of frequency, which encompass virtually the entire radio communications frequency spectrum. Accordingly, the present invention was conceived to address this important range of frequencies. The present invention makes use of concepts of the distributed RC techniques mentioned above.